Pulse wave velocity (PWV) is the speed of energy wave transmission in the arteries. According to the Bramwell-Hill equation, PWV varies inversely with the square root of arterial compliance (AC). Indeed, PWV increases as the arteries stiffen with aging and disease.
Due to the ease of its measurement, PWV has at least three potential clinical applications. First, PWV, as an index of arterial stiffness, has been shown to be an independent predictor of cardiovascular events and all-cause mortality in hypertensive and other patients. So, PWV may be used for risk stratification of, and guiding therapy in, these patients. Second, cardiac output (CO) determinations from blood pressure (BP) waveforms can only be obtained to within a scale factor that is often given by AC. Thus, PWV may be used to readily correct such determinations for AC changes and/or calibrate these determinations to absolute CO values. Third, since AC decreases with increasing BP, PWV and BP often show positive correlation. Hence, PWV may be used to realize continuous, non-invasive, and cuff-less BP monitoring.
Conventionally, PWV is measured as the ratio of the distance and pulse transit time (PTT) between proximal and distal arterial sites. PTT is, in turn, determined by acquiring waveforms from the two sites and then detecting the foot-to-foot time delay between the waveforms. The premise of this foot-to-foot detection method is that interference from the reflected wave is negligible during late diastole and early systole when the waveform feet occur. However, wave reflection interference may not always be trivial at the waveform feet. For example, at low heart rate (HR), the reflected wave adds constructively to the forward wave such that the method can grossly overestimate PWV. Just as important, the waveform feet are often difficult to detect, especially in the presence of waveform artifact. Thus, the foot-to-foot detection method is prone to error. Compounding matters, BP changes perturb PWV relatively little. As a result, even small PWV inaccuracies can lead to large errors in tracking BP. Indeed, typical plots of BP versus PWV show a great deal of scatter about the line of best fit. Such scatter obviously limits the ability of PWV to monitor BP.
Thus, there is a need for more accurate methods to determine PWV from two arterial waveforms. Such methods should be robust to artifact and avoid waveform misdetections while revealing the true PWV (i.e., the PWV in the absence of wave reflection). This section provides background information related to the present disclosure, which is not necessarily prior art.